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Organization and Significant Accounting Policies - Additional Information (Detail)			1 Months Ended	3 Months Ended
	Oct. 03, 2018 USD ($) Target	Sep. 28, 2016 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2019 USD ($)
Organization And Significant Accounting Policies [Line Items]
Proceeds from the issuance of common stock			$ 250,500,000	$ 250,476,739	$ 60,521,729
Cash and cash equivalents			461,032,249	461,032,249		$ 221,804,128
Restricted Cash			1,000,000.0	1,000,000.0
Short term investments			40,969,341	40,969,341		36,899,894
Long term investments			26,329,098	26,329,098		$ 44,175,993
Net increase (decrease) in cash and investments				225,500,000
Right-of-use assets			10,440,758	10,440,758
Right-of-use liabilities			14,838,901	14,838,901
Janssen | JNJ-3989 (ARO-HBV) Agreement
Organization And Significant Accounting Policies [Line Items]
Milestone payment	$ 25,000,000		25,000,000	25,000,000
Janssen | Collaboration Agreement
Organization And Significant Accounting Policies [Line Items]
Number of additional targets for development and sales milestone payments | Target	3
Janssen and JJDC | Collaboration and License agreements
Organization And Significant Accounting Policies [Line Items]
Cash received as due under agreement	$ 175,000,000
JJDC | Common Stock Purchase Agreement
Organization And Significant Accounting Policies [Line Items]
Proceeds from the issuance of common stock	75,000,000
Amgen | Collaboration and License agreements
Organization And Significant Accounting Policies [Line Items]
Proceeds from the issuance of common stock		$ 21,500,000
Amgen | License Collaboration and Stock Purchase Agreement
Organization And Significant Accounting Policies [Line Items]
Cash received as due under agreement		35,000,000
Proceeds from the issuance of common stock		21,500,000
Amgen | AMG-890 (ARO-LPA) Agreement
Organization And Significant Accounting Policies [Line Items]
Milestone payment		$ 10,000,000
Maximum | Janssen | License Agreement
Organization And Significant Accounting Policies [Line Items]
Development regulatory and sales milestones payments	1,600,000,000
Maximum | Janssen | Collaboration Agreement
Organization And Significant Accounting Policies [Line Items]
Development regulatory and sales milestones payments	$ 1,900,000,000
Maximum | Amgen | Collaboration and License agreements
Organization And Significant Accounting Policies [Line Items]
Development regulatory and sales milestones payments			$ 420,000,000	$ 420,000,000

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